The present invention relates to a novel drill screw which is capable of improved performance and to cutters for making it.
Many different drill screw configurations have been developed to date. Designing of these drill screws has been something less than an exact science with the reasons why some drill screws work well in some materials, but not in others, and why other drill screws do not work well at all, remaining something of a mystery. For example, it is known that for some applications, a simple nail point when turned at a sufficient rate of speed is sufficient to penetrate some materials, dry wall for example. On the otherhand, no drill screw yet devised can satisfactorily drill through some of the high strength, low-alloy steels.
Two of the basic criteria used to judge drill screw performance are; (1) the amount of end-pressure load required for the screw to drill and (2) the time in seconds for the screw to penetrate the particular material being drilled. Obviously, in an assembly line type environment where a large number of fasteners are installed by a workman in an hour's time, reduction in both the amount of end loading required and drill time will be of benefit to both the individual workman and to his employer.
Therefore, it is an object of the present invention to provide a drill screw capable of drilling with lower end-pressure load.
It is a further object to provide a drill screw that can drill in a shorter period of time even though the end load is reduced.
Further, it is an object of the present invention to provide a drill screw which is capable of drilling high-strength, low-alloy steels.
In addition, it is an object of the present invention to provide an improved method of producing a drill screw which leads to greater consistency of the product.
A further object of the invention is to provide a cutter capable of milling a drill screw flute having a chip-breaking feature.
It is yet another object of the present invention to provide a cutter for pointing a drill screw which is capable of producing a point which has a generally convex configuration. This generally convex configuration may have a uniform radius or be formed by two planes intersecting to form an obtuse included angle which is less than 180.degree., preferably about 1721/2.degree..
These and other objects of the invention are accomplished by a drill screw which has a compound flute configuration; that is, a flute which has both a straight section and a curverd section, the radius of this curved section being uniform in a plane perpendicular to the axis of the flute. The straight section includes at least a portion of the cutting edge and the curved section includes at least the trailing or drag surface. In some embodiments, the curved section also includes part of the cutting edge (defined as that portion of the leading end of the flute which is in advance of the chisel).
The drill screw of the present invention is manufactured using radiused cutters rather than the conventional fluting and pointing saws. One of the chief benefits of using radiused cutters is that, unlike conventional saws, when the teeth are sharpened, little or no material is removed from the diameter. Hence, the optimum flute configuration becomes more readily reproduced (i.e., there is less variance in quality due to wearing of the cutter). A further advantage, which also adds to part consistency, is that the teeth have a stronger configuration which is less subject to deflection.
In the manufacture of these drill screws, the fluting cutters are simultaneously plunged into the shank of the screw blank along axes which are parallel, but offset. The longitudinal axis of the cutters are inclined at equal but opposite acute angles relative to the axis of the blank as the cutters are moved along the parallel axis of movement. As a result, the flute has a straight section corresponding to the side of the cutter and a radius corresponding to the tip radius of the cutter teeth.
The drill screw may be provided with any of the conventional 90.degree. or 105.degree. point angles or, may be formed by a cutter which has a generally concave tooth configuration. This will produce a generally convex point, each portion of which may have a uniform radius of curvature or be formed by a pair of planar surfaces which intersect to form an included angle of 1721/2.degree.. In this latter instance the drill point will have a compound included angle which may be 105.degree. at the tip and 90.degree. elsewhwere.
The drill screw of the present invention has a stronger cutting edge which makes it less likely to break down when drilling the harder materials. At the same time, by maintaining a comparatively narrow chisel, the end pressure required to effect drilling can be kept to a minimum.
These and other objects, features and advantages of the present invention will be better understood by referring to the following detailed description taken in conjunction with the accompanying drawings.